AST with Go's interfaces dynamic dispatching.

main.go

package main

import (
	"fmt"
)

func main() {
	result := multiply(Value{2}, add(Value{3}, Value{4})).Evaluate()
	fmt.Println(result)
}

type Expression interface {
	Evaluate() int
}

type Value struct{ Value int }

func (l Value) Evaluate() int { return l.Value }

type Binary struct {
	Left          Expression
	OperatorToken rune
	Right         Expression
}

func (b Binary) Evaluate() int {
	switch b.OperatorToken {
	case '+':
		return b.Left.Evaluate() + b.Right.Evaluate()
	case '*':
		return b.Left.Evaluate() * b.Right.Evaluate()
	default:
		panic("Invalid operator")
	}
}

func add(a, b Expression) Expression {
	return Binary{a, '+', b}
}

func multiply(a, b Expression) Expression {
	return Binary{a, '*', b}
}